Republican rivals Mitt Romney and John McCain criss-crossed Florida by plane yesterday, dropping off at dozens of campaign stops and seeking an edge in the biggest primary yet.

Today's primary, in the fourth most populous state and one with a special resonance for Republicans, given its role in George Bush's 2000 victory, could produce a Republican frontrunner for the first time since the race began in Iowa on January 3.

A record turnout is expected that will dwarf that of the early smaller states, Iowa, New Hamphshire and South Carolina. As of yesterday morning, almost 474,000 Republicans had taken advantage of the state law that allows early voting.

With so much at stake, McCain and Romney - the moderate Vietnam war hero versus the socially-conservative multi-millionaire businessman - exchanged insults yesterday.

Romney portrayed McCain as a "liberal Democrat", tantamount to blasphemy for Republican audiences and claimed McCain had considered running alongside the Democratic presidential candidate, John Kerry, in 2004. He also put out a radio ad featuring a recording of Bill Clinton praising McCain. The personal slurs reflected an animosity between the two that has existed since Romney began running negative TV ads about McCain in Iowa and New Hampshire.

Rudy Giuliani, the former New York mayor who has put so much money and effort into winning Florida, is in third place in the polls. Mike Huckabee, who won Iowa, has almost given up on Florida because of lack of cash and is campaigning in other southern states. But the diversity of Florida, socially conservative in the north, more liberal in the south, makes it notoriously difficult to predict.

Romney began the day speaking to about 100 supporters crowded onto a small roundabout at West Palm Beach airport before heading off on his 12-hour flight to about a dozen locations.

He claimed there had been rumours that McCain had given thought to running alongside Kerry in 2004. "Had someone asked me that question, there would not have been a nanosecond of thought about it. It would have been an immediate laugh."

Romney added: "So we are different. I'm a conservative."

Among his supporters was Tad Galin, 77, from Boca Raton a US citizen who had been born in Stalin's Russia and was conscripted as an 11-year-old labourer into the German army during the second world war. He claimed that McCain, who repeatedly argues in favour of a bipartisan approach to politics in order to get legislation through Congress, had "sold out and had got into bed" with Democrats such as Ted Kennedy and Nancy Pelosi, the speaker of the House of Representatives. He also thought McCain was soft on illegal immigration.

McCain, on his tour of the state, claimed that Romney not been consistent in his politics, accusing him of being a "flip-flopper".

McCain has focused on his experience as a senator dealing with foreign affairs, contrasting this with Romney's lack of experience in the area. He claimed, erroneously, that Romney last year called for withdrawal from Iraq, in contrast to his support for sending extra US troops.

After the primary, the candidates head to California for a televised debate tomorrow before fanning out across the country in advance of next week's Super Tuesday contests that will account for about half the voting population of the US.